The problem addressed by the invention concerns stabilizing earthen material by improving its load bearing strength under conditions of soft or unstable soil and rock. Such conditions occur and are of concern primarily in the areas of building support foundations and of steep rock or soil slopes such as at a building excavation site or on the side of a hill or mountain.
It often occurs that a construction project, be it a building, a bridge or another structure, is intended to be placed on land which is made up of unstable, or weak, foundation soil. The instability may be the result of inadequate compressive strength of the surface soil, of soil subject to excess settlement, or of a firm surface layer which resides over a soft or unstable underlayer. Conditions sometimes exist where an underlayer contains a high water content which is susceptible to lateral shifting or liquefaction. These conditions may be due, in some circumstances, to land which has been built up as a result of waste deposits such as landfills, sludge beds, mine tailings or earth dredged from under a body of water.
Soil liquefaction can take place in subterranean layers of water saturated sand. When the earth is vibrated such as during an earthquake, the sand particles lose grain-to-grain contact and are reoriented and densified to the point where the water pore pressure causes the subsurface layer to act as a liquid. Since water has no shear strength, the sand layers lose all stability causing existing surface structures to immediately settle, tilt, fall on their sides, or collapse. Soil liquefaction is the single largest factor in building destruction during earthquakes.
Many methods have been devised over the years to compensate for such soil conditions. Prominent among these methods have been the practices of driving load supporting piles into the ground or constructing an oversized load supporting foundation capable of "floating" on the weak soil. Very weak soil cannot be adequately stabilized with driven piles and, at great expense, must be excavated and replaced with stable foundation materials. In the distant past, excavated soil was sometimes heat treated in processing ovens and returned to the same site in a more stable condition. This procedure is not considered economically viable today.
Driving piles into the ground is a long process, requiring a large number of piles to be placed in close proximity so as to substantially supplement the weight supporting ability of the soil. A long-term danger exists with the method of inserting piles because over the life of a building or bridge, for example, the piles might decay, if made of wood, or rust, if made of metal. A pile made of concrete avoids the decay or rust problem, but tends to be more difficult to drive into the soil. Irrespective of the material of which the piles are made, the driven piles are forced into the ground until they either reach bedrock or the force required to drive exceeds a predetermined maximum. Since the depth to which a pile being driven into a particular soil cannot be predicted, some length of pile will be left above the ground. This excess length must be cut off flush with the foundation level so a flat surface will be available on which to build. A variation which has reduced the prior difficulties of driven piles involves cast-in-place concrete piles. The method of installing artificial piles requires a large number of piles if a substantial weight is to be supported and involves significant material and installation cost. Only piles which extend to bedrock will guarantee a firm structural foundation. The foundation stability of piles not able to reach bedrock could be compromised by weak underlying layers or by a change in soil properties such as caused by an increase in water content.
The alternate traditional solution for supporting a load on soft surface soil has been that of utilizing a "floating" foundation. Floating is accomplished by excavating a hole that is larger than the proposed building base and constructing a greatly oversized foundation. By effectively spreading the weight being supported over a large area, the tendency to settle is reduced. The floating foundation method requires an expenditure of considerable material and it is not always feasible to extend the dimensions of a foundation because of lack of available land or because of unsuitable topography. Again, the post-construction conditions outlined above in pile foundations could also threaten the stability of floating foundations.
A further method of foundation soil stabilization known in the construction industry is that of in-situ thermal hardening. To accomplish thermal hardening, a hole is drilled in the soil and the earth surrounding the hole is heated by oil or coal fuel so as to first drive off the moisture and later cure the soil to a tubular pile of brick-like hardness. This method is best suited to clay or loess based soils and tends to be very slow. A single prior art thermally hardened tubular pile sometimes requires several weeks to be formed.
A second aspect of the problem being described is that of correcting for differing rock and soil conditions on relatively steep inclines, such as found on a hill or mountain side. A mountain is generally made up of not one rock or soil, but of a plurality of individual rock and soil sections. There is often a layer of soil or of rock on the surface which is resting on underlying rock at a subterranean interface. When rain falls on the mountain, some of the water may find its way into the ground at the uphill place where the layer interface comes to the surface. The water continues to travel underground along the interface. The imposition of water at the interface creates a lubricated "slip plane" condition which, if the overlying layer is not prevented from sliding, for example, by an upwardly directed rock outcropping, could allow the overlying layer to slide down the mountain as a landslide. Similarly, slip planes can form in homogeneous earth masses which become unstable due to saturation, earthquake or other causes.
Modern geological testing methods allow the determination of the location, depth, and configuration of such slip plane interfaces. By locating earth masses susceptible to landslides, intervention can be accomplished before slides actually occur.
In the landslide conditions outlined above, remedial measures taken heretofore have been relatively expensive and not uniformly successful. A commonly known corrective method involves the construction of a protective retaining barrier at the base of a slope so as to prevent damage if a slide does begin. It has also been known to drill holes in the face of a mountain through its outer earthen mass and through the slip plane interface, pump concrete grout into the holes and then allow the grout to harden in order to stabilize the earthen mass. However, this system is very costly and has many disadvantages, e.g., the possibility of underground water reducing the concentration and strength of the concrete grout mix or the inability to determine exactly where the mix flows beneath the surface.
The invention disclosed herein recognizes that there exists a relatively new technology which may be employed in remediation of unstable land masses by the application of very high quantities of heat energy. The basic tool used in this technology is the plasma arc torch. Plasma torches can routinely operate at temperatures of 4000.degree. C. to 7000.degree. C. in the range of 85-93% electric to heat energy efficiency. The highest temperature attainable by combustion sources is in the vicinity of 2700.degree. C.
A plasma arc torch operates by causing a high energy electric arc to form across a stream of plasma, or ionized gas, generating large amounts of heat energy. There are many types of plasma torches, but all torches generally fall into one of two basic categories according to the arc configuration relative to the torch electrodes, i.e., transferred arc type and non-transferred arc type. The arc of a transferred arc torch is formed by and jumps from a single electrode on the torch, through the gas, and to an external electrode which is connected to an opposite electrical pole. The arc of a non-transferred arc torch is formed by and jumps from one electrode on the torch across the plasma gas to another electrode on the torch.
In a plasma arc torch, the heat energy produced is proportional to the length of the arc, assuming an identical plasma gas at a uniform flow rate and a constant applied electrical current.
Since the present invention makes use of a plasma arc torch, reference is next made to U.S. Pat. No. 4,067,390 granted to the present inventors for "Apparatus And Method For The Recovery Of Fuel Products From Subterranean Deposits Of Carbonaceous Matter Using A Plasma Arc" which teaches the use of a plasma arc torch to gasify or to liquify underground deposits of coal oil shale and other carbonaceous materials. As with the method of the present invention, the method of the patent involves lowering a plasma arc torch into a hole and using the torch to create heat in the carbonaceous matter. In the method of the patent, the heat is used to gasify or liquify underground carbonaceous deposits and potential subsidence of the deposit overburdens is avoided by leaving pillars of earth at intervals for support. Also to be noted for further background is that the method of the patent involves monitoring selected properties of the fuel products and using the measured fuel properties as a means for adjusting the torch position at the base of the hole.
Having described the background art, the description next provides a summary of the invention and is followed by a more detailed description of the invention from which the differences between the method and apparatus of the invention and the prior art practices will become readily apparent.